Various methods have been used in the past to construct precast walls for retaining earth, soil, sand or other fill generally referred to as soil. As typical precast wall system is disclosed in U.S. Pat. No. 4,914,876 assigned to the Keystone Retaining Wall Systems Inc. by Paul J. Forsberg. The Keystone Patent illustrates a typical modular block wall system wherein the wall face is comprised of concrete masonry units connected to the geosynthetic wall reinforcement layers. The geosynthetic tensile inclusion members for this type of retaining wall structure are typical referred to as "geogrids".
A disadvantage of such a system is that a considerable amount of hand labor is required to install the numerous small block facing units. This limits the amount of wall structure that can be completed in any work shift. In addition, if the wall is placed on weak foundation soils, the manifestation of wall settlement is cracking or more significant crushing or crumbling of the facing units. If the settlement is excessive, the geogrid material can be sheared at the concrete masonry unit horizontal joints which can result in wall failures.
Numerous other types of concrete block mechanically stabilized earth (MSE) wall systems are available. All of these products, such as the Keystone wall type previously described, mandates precise grading and compacting of the wall backfill to correspond to increments of the vertical height of the block facing units so that the tensile inclusion material used to mechanically reinforce the retained wall backfill material will be at the horizontal joint elevation of the concrete masonry units. Although the material costs for these types of wall systems are low, due to the high labor costs of various stages of the wall construction for the these systems the resultant installed price of walls constructed with these products can be substantially higher than the material costs.
Another broad range of mechanically stabilized walls include walls that use precast concrete panels for the wall facing elements such as walls that utilize components provided by the Reinforced Earth Company of Arlington VA. U.S. Pat. No. 4,961,673 issued to Pagano et al. along with U.S. Pat. Nos. 3,421,326; 3,686,873; 4,0425,965 and 4,116,010 to Vidal describe such a wall system. Wall systems such as the Reinforced Earth products and those of the VSL Company, U.S. Pat. No. 4,725,170 by Edgar Davis, require the use of metal reinforcing strips or steel grids to be used as soil inclusion members in the wall backfill and to be connected to the precast wall panels to hold the panels in place and to provide stability for the wall backfill.
All of these types of wall systems require that the facing panels be placed on a continuous cast in place leveling pod. The elevation of the foundation pad for these systems corresponds to the base elevation of the MSE wall structure. The base of all retaining walls, either cast- in-place or MSE, is typically required to be depressed with respect to the final: grade in the front if the wall for geotechnical stability or for frost protection. Heretofore all wall systems currently in use have a bottom of wall facing elevation that corresponds to the bottom or base elevation of the MSE reinforcement elevation. In addition the facing elements of these systems are required to be installed simultaneously with the placement of the wall backfill and soil reinforcement.
A disadvantage of MSE walls that use metal soil reinforcement is that the metal soil tensile inclusion members used are subject to corrosion since the metal is in direct contact with the wall backfill. Numerous catastrophic failures have resulted from the effects of unchecked corrosion on the metal tensile inclusion members for these types of wall systems. Although the metal strips or steel grids can be galvanized to reduce the effects of the oxidation process this technique is not effective for all soil types due to the diverse mineral content present in some soils. Other methods such as epoxy coating for the metal soil reinforcing members have been used to further resist the deleterious effects of potential chemical reactions of the minerals present in the soil in contact with the soil reinforcement. A disadvantage of the epoxy coating is that the coating is easily scratched during the construction process which result in the exposure of the steel or metal soil reinforcement to the corrosive effects of the minerals present in the backfill. Also, epoxy coatings increase the costs of these systems.
Since the wall facing components in all precast panel or concrete masonry unit wall systems currently in use are installed simultaneously with the wall backfill, another disadvantage of these systems, besides the need for close backfill placement tolerances, is the fact that a portion of the soil mass adjacent to the wall facing units does exert a horizontal force on the face.
Typical wall facing units for existing MSE systems in current use may range in size from 8".times.16" for block systems to 25 to 50 sq. ft. for precast panel wall systems. The concrete masonry block systems, due to the high unit weight and relatively small size of each block, do not require bracing or interlocking to hold the face units in a vertical position as the wall backfill is placed. Since the blocks are heavy (exceeding 100 pounds for some applications) the placement of the blocks is physically demanding which adds to the placement cost of the facing units. For currently available MSE wall systems that use panels for facing units the panels are large in size compared to the block facing units and the panels (typically between 25 to 80 sq. ft. in area) are held in place during backfilling operations by interlocking with the previously placed or adjacent panels. For some systems the facing units are "wedged" or leaned by other methods so that the effect of the interaction of the backfill pressure and the metal soil reinforcement will, in theory, force the panels into a plumb or vertical position. Panel placement for these systems require skilled experienced workers to erect the units so that the resultant structure will be vertical and not leaning either in or out of a vertical plane.
Full height panels have been used on MSE walls where the MSE layers are connected to the wall face. Temporary erection braces are required for these system to hold the panels in place as the backfill is placed behind the wall. This requires additional working right-of-way in front of the wall and restricts site access. Since the soil reinforcement material, whether geosynthetic or metal, is not designed for concentrated high loads at the connections of the soil reinforcement material it is critical that all panel connections should, in theory, have quantifiable uniform loads. This condition is extremely difficult, if not impossible, to achieve in the field. This is one of the primary reasons why few full height MSE panel walls have been built with precast face units. An indeterminacy situation exists for the load determination at the numerous connections of the soil reinforcement material to the panels for these types of walls since typically the number of soil reinforcement connections to the wall facing exceeds the number of equations available to solve for the individual connection loads.
There is a portion of retained soil loading on the wall face in full height and all MSE panel systems currently in use and any vertical settlement (relative motion) between the tensile inclusion soil reinforcement layers and the panel face can induce excessive shear loads on the soil reinforcement material at the connection point to the panel. Typically there is no adequate provision to allow for this vertical movement without inducing shear forces on the tensile inclusion material at the connection to the wall face for the systems currently in use. Many panel connection devices have been installed and utilized for these various systems currently in use wherein the wall face can, in theory, move with respect to the soil reinforcement material. Panel connections such vertical bolts supported by clevises cast into the panels connected to the metal strip soil reinforcement have been used to allow for vertical movement. The high horizontal earth loading on the individual connections results in large friction loads at the bolt and as a result the relative motion desired at the connection has not typically been achieved.
Vertical settlement of the whole MSE wall mass, wherein the panels move with the MSE structure is, for some sites a valid assumption, because the forces supporting the vertical wall and backfill loads are uniform. Unfortunately, for certain wall sites, the retaining structure may rest on material that does not have uniform bearing capacity over the reach of the wall. For these sites, if there is compressible material under some portions of the MSE mass, the structure will not settle uniformly. This can result in differential settlement between the wall elements and the wall mass which can lead to structural failures of varying degrees.
Another broad range of MSE wall types that have been used extensively for permanent and temporary retaining wall applications are wrapped face or confined fill layers that form the geotextile MSE wall. These walls are comprised of an assembly of vertically stacked layers of wall backfill confined by closed face sheets of geotextile that are typically placed in horizontal planes within the wall backfill as the backfill is placed and compacted. For temporary walls the the face of these walls is the exposed geotextile material. The geotextile that retains the fill at the face of each layer is wrapped back into the fill behind the face of the wall. The wrap back geotextile is imbedded into the backfill material behind the face of the wall for each compaction lift of fill that is placed. One of the difficulties associated with the construction of these types of earth retention structures is that the wrapped back face portion of each backfill layer requires that an external forming system be installed in front of the face of the wall to hold the geotextile face at the proper alignment until the wrap back portion of the geotextile layer is sufficiently imbedded in the backfill adjacent to the wall face. The associated fill pressure prevents the wrap back geotextile from being displaced horizontally. The cost of labor associated with the placement and operation of the external forming system adds to the cost of these types of walls.
Whether the geotextile wall is a temporary or permanent structure a face forming system is required so that the resultant overall wall face will conform to the wall alignment limits. For permanent geotextile walls it is necessary to cover the exposed wall face so that the geotextile will be protected from the deleterious effects of prolonged exposure to ultra violet radiation. Although the geotextile material is corrosion resistant with respect to the soils and minerals that the material may come into contact with due to the embedment in the wall backfill the long term effects of exposure to the sun can result in the ultimate deterioration of the wall face. Various facing materials that have been used to cover the face of geotextile walls include: sprayed concrete faces, precast or cast in place concrete panels. The use of a sprayed concrete face require that attachment fasteners such as lengths of wire or pieces of rebar be installed in the wall and protrude from the face of the wall to form a connection between the sprayed on concrete and the exposed geotextile surface. The disadvantage of walls with this type of face is that the wall surface is typically not uniform and not aesthetically pleasing. Additionally if the walls experience any significant long term settlement cracking and spalling of the sprayed concrete face can occur.
Precast facing elements have also been attached to wrapped face geotextile walls by the use of long bolts or thread bar anchors that are screwed into the geotextile earth retention structure. Although these methods are adequate to provide U.V. protection because of the metal anchors the life of the wall is reduced. Also the precast facing is rarely attached accurately so the resultant wall face may not be uniform in appearance.
Another wall face that has been used for geotextile walls is the option of casting a poured in place concrete face over the geotextile wall. This approach can result in a uniform aesthetic face but it does require extensive forming and the associated high field labor and material costs. These additional costs can make walls of this type less competitive than other conventional wall types.
In view of these and other shortcomings of prior art, there is a need for an improved MSE retaining wall system. Accordingly it is the object of the present invention to provide an improved MSE wall system with a full height panel facing of precast concrete or other suitable material that can be precast, pre-manufactured, or assembled and that, although attached to a separate structural MSE wall ,the face is isolated from the MSE wall.
It is another object of the present invention to provide an improved MSE wall retaining system wherein the reinforced soil mass can be constructed to essentially full height prior to attaching the wall facing units.
It is another object of the present invention to provide an improved MSE wall retaining system wherein the reinforced soil mass is comprised of layers of confined soil, sand, or other suitable backfill material for use in MSE walls.
It is a further object of the present invention to provide for an improved wall system that can utilize a mechanically stabilized backfill wall formed by vertically stacked confined fill layers of flexible tensile inclusion soil reinforcement material and wall backfill.
It is another object of the present invention to provide an improved MSE wall retaining system wherein tie rods and anchors are installed in the reinforced soil mass formed of confined fill layers as it is built.
It is a further object of the present invention to provide for an improved wall system that utilizes connecting tie rods that exhibit a low sliding coefficient of sliding friction between the confined fill layers of the separate MSE wall.
It is a further object of the present invention to provide for an improved wall system that can utilize full height wall panels that are connected to tie rods previously placed in the layered confined fill MSE wall.
It is a further object of the present invention to provide for an improved wall system that allow the use full tier height wall facing units where the top of the wall panel or facing unit corresponds to, at a minimum, the top of the separate MSE wall.
It is a further object of the present invention to provide for an improved wall system that allow the use full tier height wall facing units that if the top of the panel corresponds to the top of the overall wall height that the panel height is less than the overall height of the wall.
It is still a further object of the present invention to provide an improved wall system that prevents significant earth loading to be transmitted to the facing units.
It is yet another goal of the present invention to provide an improved wall system that can utilize tensile inclusion members of either geosynthetic, metal or other suitable flexible, high tensile strength material.
It is yet another object of the present invention to provide an improved wall system that allows the facing units to be placed on a wall panel foundation that is located at a higher elevation than the base of the confined fill layers of a separate MSE wall.
It is still a further object of the present invention to provide for an improved wall system that provides a work platform within the separate MSE structure at an elevation substantially above the base of the wall to facilitate the installation of the facing panels.
It is still a further object of the present invention to provide for an improved wall system that allows the placement and attachment of the full height facing units to the separate MSE wall from the work platform on the wall without the use of temporary erection braces.
It is still a further object of the present invention to provide for an improved wall system that allows the facing units and or the facing unit support components to function as a face form support for the confined fill layers of the separate MSE wall.
It is still a further object of the present invention to provide for an improved wall system that does not preclude the confined fill soil layers of the separate MSE wall to be installed parallel to the grade at the top of the wall.
It is still a further object of the present invention to provide for an improved wall system that has a minimum number of connections to each wall panels from the MSE soil mass.
It is still a further object of the present invention to provide for an improved wall system that provides a continuous void space between the facing units and the face of the confined fill layers of the separate MSE wall.
It is still a further object of the present invention to provide for an improved wall system that utilizes continuous spanning closure components at the upper portion of the separate MSE wall to span the horizontal void between the back of the facing units and the face of the confined fill layers.
It is still a further object of the present invention to provide for an improved wall system that allows the optional use of compressible chimney fill to partially fill the void space between the back of the facing units and the front of the layers for the confined fill MSE as a compressible layer to compensate for horizontal strain within the MSE mass and not transfer these stresses to the facing units.
It is still a further object of the present invention to provide for an improved wall system to have sufficient tolerances at the component connections to allow for significant vertical and horizontal displacement at the component interface to facilitate ease of assembly of the components.
It is still a further object of the present invention to provide for an improved wall system that reduces or effectively eliminates the possibility of inducing vertical shear stress on the facing unit connectors to the separate M.S.E. earth retention structure.
It is still a further object of the present invention to provide for an improved wall system that is not dependent on the type of MSE soil reinforcement used for the soil tensile inclusion members for the separate MSE wall.